Vapor deposition and sputtering processes are known techniques for depositing very thin layers, commonly termed "films", of various materials on substrates. The thicknesses to which such films are deposited are often measured in Angstrom units (1 Angstrom unit equals 10.sup.-10 meters). Typically, films deposited by sputtering and vapor deposition processes have thicknesses in the order of several hundred to several thousand Angstrom Units. The present invention will be described with respect to a typical, closed chamber sputtering process as a preferred embodiment. However, it should be understood that such description of a specific apparatus is only intended to highlight features of the invention without limiting the scope and application of the invention to the described apparatus.
In the electronics industry, sputter-deposited films are sometimes used to provide metallization patterns for semiconductor devices. Sputter-deposited thin films are also used to provide on substrates, resistive films which are further processed into precision resistors and into conductors on hybrid integrated microcircuits.
In such functional film depositions, as opposed to the deposition of decorative films on dinnerware, for example, not only the precise thickness of a film, but also its quality, namely its density, is of significance. When a deposited film is analyzed by scanning electronmicroscopy, for example, differences in the microstructures of various deposited films can be observed.
The surface of a film may appear smooth and continuous, even though only a relatively thin film, compared to other sputter-deposited films, may have been formed. On the other hand, a comparatively thicker film may show a ragged surface texture as a result of a loosely deposited and porous underlying film structure. A continued sputter deposition on such a film may ultimately yield a smoothly textured, continuous film surface; however, a layer of relatively greater thickness may be necessary to first obtain a layer of a homogeneous microstructure in this latter example.
If a deposited film is to be used as a base material for a conductor or resistor in a microcircuit, strict control over the microstructure in such film is likely to effect production yields. Often such films are formed to a much greater thickness than is necessary to overcome any inconsistencies in the quality of their deposition, as may be caused by inadvertent changes in one or more of any number of deposition process parameters.
One of these parameters which is likely to affect the quality of a deposited film of material is the quality of the underlying surface, typically a highly polished substrate on which the film is deposited. In addition to the quality or surface smoothness of such substrate, the film may be affected by various other apparatus and process parameters.
In a sputtering operation, for example, a chamber is substantially evacuated to an absolute pressure in the range of 10.sup.-2 torr. A gaseous ion source such as argon remains in the chamber between a cathode target and an anode. An electric discharge between the cathode and anode ionizes the gas, and a DC bias directs the ions against the target of the material to be sputtered. Material is eroded from the target by the impinging ions and flies off toward a workholder where it deposits on the workpieces or substrates as a film.
From this brief description of a typical sputtering operation it may appear that several variables such as pressure, temperature, electrical fields and material selections interact, and each of these variables may be modified either by intent or accidentally. Any change in any of the interrelated variables or parameters may, however, bring about a change in the microstructure of the film that is being deposited. At times, the significance of a changed parameter and the effect of such change on the quality of a deposited film may not even be appreciated.
Changes in the film quality may be brought about by a change in the process temperature, by vacuum changes, by changes in the ion density, or by whether or not AC high frequency enhancement currents are applied. Magnetic enhancement fields are often used in sputtering operations. A change in the magnetic field changes current flow and ion flow patterns within the chamber. Such changes are likely to affect the impact speed of deposited material. Consequently, a deposited film of optimum quality is apt to be obtained only when all of these interrelated factors are optimized with respect to each other.
It therefore appears to be highly desirable to have an in-process indicator to verify whether a particular film which is being deposited is of a desired quality or whether a change has occurred, resulting in a film of a quality different from that of a prior lot. When a change in the quality of a film is detected, it may at that point be too late to change some of the parameters of the ongoing process. However, an early indication of a change in the quality of the microstructure of a film may be the only opportunity to detect otherwise hidden defects which ultimately tend to affect the useful life of the product.